Bronchopulmonary dysplasia is a chronic respiratory disorder of infants characterized by persistent respiratory distress and the prolonged need for supplemental inspired O2. A central problem that plagues nearly all of these infants is their inability to thrive in the presence of increased metabolic demands and often limited nutritional intake. It is the aim of this proposal to address the respiratory factors that influence the tenuous energetic balance in the premature infant with protracted respiratory distress leading to chronic lung disease. We have hypothesized that four components of work (elastic, viscous, viscoelastic and distortional) performed during respiration all contribute to determine the ventilatory pattern and the need for particularly important in infants with chronic lung disease. We have also hypothesized that respiratory pump inefficiency specifically caused by alterations in diaphragmatic configuration, failure of respiratory muscle recruitment, chest wall distortion, and poor nutrition raises metabolic demands and contributes to respiratory and growth failure. These hypotheses will be addressed in premature infants with respiratory distress syndrome who require mechanical ventilation for at least 14 days and either develop chronic lung disease or have resolution of their respiratory distress. Measurements of work done on the lungs and the chest wall, and calculations of the energy expenditure to perform this work as the ventilatory load is altered will form the basis of testing the first two hypotheses. These data should yield important new information related to: 1) the factors which contribute to respiratory failure in infants with chronic lung disease, 2) the mechanisms for respiratory pump inefficiency, and 3) therapeutic strategies for managing growth failure and respiratory muscle dysfunction in early infancy.